Microlithography has been widely used in the electronics industry for more than twenty years. With the rapid growth of the electronics/information industry and the continuous improvements in the technologies for manufacturing VLSI circuitry and color filters for liquid crystal displays, today microlithography plays an even more important role, and it is important to devote research and development effort for improved microlithographical technology.
One of the key elements of microlithography is the photoresist composition. A photoresist composition comprises a coating composition produced from a solution or applied as a dry film which, when exposed to light, will be chemically altered in its solubility to certain solvents, or the so-called "developers". The are two main types of photoresists, negative photoresists and positive photoresists. The negative photoresist is a mixture which is initially soluble in a developer, but upon exposure to light, becomes insoluble in that developer to thereby form a latent image. A positive photoresist, on contrast, is a mixture which is initially insoluble in the developer. Upon exposure to light, the photoresist becomes soluble in the developer to form a positive image. Positive photoresists are typically more expensive than negative photoresists. However, because positive photoresists can produce images with substantially better resolution and possess superior heat and oxidative resistances, they are more suitable for use in the manufacturing of high density microelectronic components using microlithography, and have thus become the mainstream material in related fields.
A photoresist composition typically comprises a photosensitive compound dispersed in an appropriate polymer binder. The most commonly used polymer binder is a phenolic resin, which is a condensation copolymer of phenol(s) and formaldehyde. The phenolic resin is soluble in many organic solvents as well as in basic solutions. With regard to photosensitive compounds, many quinonediazide compounds have been used for this purpose to form positive images. These quinonediazide compounds will be chemically converted into carboxylic acids upon exposure to light rays of appropriate wavelengths. The resultant carboxylic acids are substantially more soluble in basic solutions than the original unconverted compounds. Thus when the wafer or any substrate that has been coated with a film containing such a positive photosensitive compound is treated with a basic solution (i.e., a developer), the exposed regions will have a higher rate of dissolution than the unexposed regions. Such a difference in the rate of dissolution in a basic solution causes the desired images to be developed.
The quinonediazide compounds are the most widely used positive-imaging-forming photosensitive compounds. Examples of these quinonediazide compounds include esters 1,2-naphthoquinone diazide-5-sulfonyl chloride or 1,2-naphthoquinone diazide-4-sulfonyl chloride with trihydroxybenzophenone. These and other quinonediazide compounds suitable for use in making positive photoresists are disclosed in U.S. Pat. Nos. 3,046,118; 3,148,983; 3,402,044; 4,115,128; 4,173,470; 4,550,069; 4,551,409; Laid-Open Japan Patent Application Nos. 60-134,235; 60-138,544; 60-143,355; 60-154,248; and European Patent Application No. 0092444. The contents of these patent and patent applications are hereby incorporated by reference.
Positive photoresists have also been used in electrodeposition lithography, which involves combining the techniques of microlithography and electrodeposition of color filter resins to produce color filters for use in color liquid crystal displays. Discussions of these processes have been described in U.S. Pat. Nos. 5,214,541 and 5,214,542, the contents thereof are incorporated herein by reference. In these applications, a positive photosensitive coating film is formed on a transparent electrically conductive substrate. The photosensitive coating film is exposed to light through a mask having patterns of at least three different degrees of light transmittances with respect to light rays of different wavelengths, typically of the red, green and blue light colors. Then a portion of the photosensitive coating film is developed and removed to register with one of the patterns of smallest and largest degrees of light transmittances for exposing the transparent eclectically conductive layer. Thereafter, a colored coating is electrodeposited on the exposed electrically conductive layer for forming a colored layer thereon. The steps of developing and removing the photosensitive coating film and electrodepositing the colored coating are repeated for the respective patterns of different degrees of light transmittance in sequence of difference in light transmittances to form different colored layers. These steps for forming the color filters using a positive photoresist is summarized below:
______________________________________ forming a photosensitive film .arrow-down dbl. exposure to light irradiation .arrow-down dbl. development .arrow-down dbl. electrodeposition (of the first color) .arrow-down dbl. development .arrow-down dbl. electrodeposition (of the second color) .arrow-down dbl. development .arrow-down dbl. electrodeposition (of the third color) ______________________________________
One of the main advantages of the multi-color lithographic/electrodeposition process described above is that only one photomask is required and the photosensitive compound only has be exposed to the light source once. However, the photoresist that can be used for this process must exhibit an ability such that it can be successively developed by a developer solution of increased strengths or concentrations, so as to allow multi-colors to be electrodeposited on the transparent substrate.
Because the dimensions of the commercial electronic equipment are getting smaller everyday, increased precision is required of the micro-electronic components. This imposes a severe demand on the IC manufacturers to ensure the exactness of the microlithographic technology. In order to satisfy this need, the photoresist must provide high resolution and can be developed in a relatively quick manner. Most of the photoresists do not provide the kind of resolution and speed that will satisfy today's stringent requirement. Furthermore, the photoresists that are currently available require relatively strong basic solution, at least at the level of 2.38% tetramethyl ammonium hydroxide, for image development. The high alkalinity required of the developer solution can be deleterious to the lithographic process and high consumptions of exposure energy and developer solution.
U.S. Pat. No. 4,731,319, the content thereof is incorporated by reference, discloses a phenolic polymer binder for use in photoresist prepared from copolymerization of isomers of meta- and para-methylphenols and formaldehyde to obtain improved heat-resistance. Japanese Laid-Open patent publication 60-164,740 further included 3,5-dimethylphenol to the copolymer composition to increase the softening temperature thereof. Other references, the contents thereof are incorporated by reference, also discuss using other improved phenol-formaldehyde copolymers for various purposes. These include: U.S. Pat. No. 4,719,167, which discloses the use of a mixture of meta- and para-methylphenols and 2,5-dimethylphenol in preparing the phenol-formaldehyde copolymer; European Patent Application No. 0496640 (1992) discloses the use of a mixture of meta-methylphenol, 2,3-dimethylphenol, and 3,4-dimethylphenol in the preparation of the phenol-formaldehyde copolymer; WO91/03769 discloses the use of a mixture of 2,3-dimethylphenol and 2,3,5-trimethylphenol in preparing the phenolic resin.
These polymer binders increased the stiffness of the polymeric structure so as to obtain improved heat resistance and improved contrast. However, they do not address the problems discussed above. For example, these polymer binders can only be used in single-exposure single-development micro-lithographic applications, and are not suitable for making color filters via multiple image development. Also, because the main objectives of these polymer binders are to improve the heat resistance and etching resistance of the polymer binder, these polymers possess a relatively stiff molecular structure. As a consequence, a highly basic solution, typically a 2.38% tetraammonium hydroxide, is required for image development. The need for a strong basic developer solution imposes several problems especially when they are to be used as a photoresist component in making color filters (for color liquid crystal displays) using electrodeposition techniques. Typically a cationic resin is used in the electrodeposition process to provide the color films. Before they are hardened, these electrodeposited anionic resins contain acid groups, and the strong basic developer solution often causes the color films to be peeled off from the substrate. Thus the use of a strong basic developer solution severely limits the type of electrodepositing solution that can be used in the electrodeposition process.
In 1991, D. N. Khanna presented a paper which disclosed the use of a mixture of meta-methylphenol, 3,5-dimethylphenol, and resorcinol in the preparation of a phenolic resin (SPE "Polymer Session," p91-111, 1991). The photoresist made from the phenolic resin exhibited both improved heat resistance and improved image developing speed. However, this phenolic polymer binder is suitable only in providing submicron resolutions, and, because it is only suitable in the traditional lithography, it is not suitable for making color filters for color liquid crystal displays, which involve line width in excess of 20 .mu.m.